a) Field of the invention:
The present invention relates to an optical waveguide device and method of making the same which are to be used in the fields of optical communication, optical information processing and so on.
b) Descripruion of the prior art:
Since ferroelectric crystals such as LiNbO.sub.3, LiTaO.sub.3 and KTiOPO.sub.4 exhibit remarkable electrooptical effects, accoustooptical effects and non-linear effects, there have been developed devices for controlling light with optical waveguides formed on base plates made of these crystals. The optical waveguide device using the LiNbO.sub.3 crystal is studied most widely. However, the optical waveguide formed on a base plate made of the LiNbO.sub.3 crystal produces, like the optical waveguide formed on the bulk crystal, a phenomenon that the index of refraction thereof is changed by an intense incident light (optical damage). The optical damage reduces a differnce in index of refraction between the optical waveguide and that of the region surrounding the optical waveguide, thereby posing a problem in that light leaks out of the optical waveguide. In recent years where it is demanded to develop optical waveguide devices compatible with intense incident lights, and since the optical damage becomes much greater as irradiating lights have higher intensities, it is desired to further reduce the optical damage.
The mechanism which produce the optical damage is qualitatively considered as described below. A sectional view of the optical waveguide region of a conventional optical waveguide device is illustrated in FIG. 1, wherein the reference numeral 1 represents a base plate made of a ferroelectric crystal, and the reference numeral 2 designates an optical waveguide formed as a region having a high index of refraction in a space including the surface of the ferroelectric crystal base plate 1. When carriers (electrons, etc.) are excited by an irradiating light from the impurity level to the conduction band in the optical waveguide 2, they are moved under the ferroelectric effect for a certain distance in the direction from -C to +C, and then protected by the defects such as impurity and holes in the crystal base plate 1, thereby producing a spatial electric field E.sub.sc [V/cm] between the optical waveguide 2 and the crystal base plate 1. It is said that the optical damage is produced due to the change of the index of refraction (lowering of the index of refraction) of the optical waveguide 2 which is induced by said spatial electric field. The change of index of refraction .DELTA.n of the optical waveguide 2 is expressed as follows: EQU .DELTA.n=-(1/2)n.sup.3 r E.sub.sc ( 1)
wherein the reference symbols n and r represent the index of refraction and the electrooptical constant [cm/v] respectively of the crystal composing the optical waveguide 2. Further, said spatial electric field is expressed as follows (see R. A. Becker and Williamson, Appl. Phys. Lett. 47, 1042, 1985): EQU E.sub.sc =.alpha.K I.sub.ir /(.sigma..sub.d +.sigma..sub.ph) (2)
wherein the reference symbols .sigma..sub.d, .sigma..sub.ph, .alpha.and K represent the electrical conductance [.OMEGA..sup.-1, cm.sup.-1 ], the photoconductivity [.OMEGA..sup.-1, cm.sup.-1 ], the optical absorption coefficient [cm.sup.-1 ] and the glass constant [cm/v] respectively of the above-mentioned crystal. Further, the reference symbol I.sub.ir designates the intensity of the irradiating light [W/cm.sup.2 ].
In order to reduce the optical damage, examinations are currently made mainly on the two methods which are described below. One method is to weaken the above-mentioned spatial electric field by increasing the photoconductivity .sigma..sub.ph which increses in proportion to the intensity of irradiating light I.sub.ir, the electrical conductance .sigma..sub.d being far lower than the photoconductivity .sigma..sub.ph when the intensity of irradiating light I.sub.ir is high. Accordingly, the electrical conductance .sigma..sub.d can be omitted in the formula (2) and it is transformed as follows when the intensity of irradiating light I.sub.ir is high: EQU E.sub.sc =.alpha.K I.sub.ir /.sigma..sub.ph ( 3)
Hence, it is considered from the formula (3) that enhancement of the photoconductivity .sigma..sub.ph is effective for weakening the spatial electric field E.sub.sc. As a method to enhance the photoconductivity .sigma..sub.ph, there is known the Czochralski process which grows the crystal by adding approximately 5 mol % of MgO to a melt of LiNbO.sub.e and it has been reported that this method permitted increasing the photoconductivity .sigma..sub.ph approximately one hundred times as high as that of LiNbO.sub.3 crystal to which MgO was not added (see D. A. Bryan et al, Appl. Phys. Lett. 44, 847, 1984).
The other method is based on a fact that the optical damage is originally caused by excitation of the carriers due to the light absorption mainly by impurities, and attempts to grow the crystal as pure as possible by reducing the amounts of the impurities to be contained in the crystal.
However, it has been reported that when the optical waveguide 2 is formed by diffusing Ti metal in the LiNbO.sub.3 crystal to which MgO is added by the first method, the photoconductivity .sigma..sub.ph of the optical waveguide 2 was the same as that of the optical waveguide 2 formed by diffusing the Ti metal in the LiNbO.sub.3 crystal to which MgO was not added (see R. A. Becker, SPIE Vol. 587, 12, 1985). Accordingly, the LiNbO.sub.3 crystal to which MgO is added reduces the optical damage in the state of the bulk crystal, but cannot reduce the optical damage in the optical waveguide 2 which is formed by diffusing Ti metal therein. Further, the high purification of the crystal by the second method cannot be attained without improved techniques for refining of raw materials, cleaning of the growing environment, etc., and the effect of the second method cannot be expected to be obtained in the near future.
As is understood from the foregoing description, there has been found out no means as yet for significantly reducing the optical damage in the optical waveguide formed on the base plate 1 made of the ferroelectric crystal.